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>8.1. An overview of boots and shutdowns</A
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><P
>The act of turning on a computer system and causing its
	operating system to be loaded
	
		<A
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HREF="#FTN.AEN1890"
>[1]</A
>
		
	is called <I
CLASS="GLOSSTERM"
>booting</I
>.  The name comes from
	an image of the computer pulling itself up from its bootstraps,
	but the act itself slightly more realistic.</P
><P
>During bootstrapping, the computer first loads a small piece
	of code called the <I
CLASS="GLOSSTERM"
>bootstrap loader</I
>, which
	in turn loads and starts the operating system.	The bootstrap
	loader is usually stored in a fixed location on a hard disk
	or a floppy.  The reason for this two step process is that
	the operating system is big and complicated, but the first
	piece of code that the computer loads must be very small (a
	few hundred bytes), to avoid making the firmware unnecessarily
	complicated.</P
><P
>Different computers do the bootstrapping differently.
	For PCs, the computer (its BIOS) reads in the first sector
	(called the <I
CLASS="GLOSSTERM"
>boot sector</I
>) of a floppy or
	hard disk.  The bootstrap loader is contained within this sector.
	It loads the operating system from elsewhere on the disk (or
	from some other place).</P
><P
>After Linux has been loaded, it initialises the hardware and
	device drivers, and then runs <B
CLASS="COMMAND"
>init</B
>.  
<B
CLASS="COMMAND"
>init</B
>
	starts other processes to allow users to log in, and do things.
	The details of this part will be discussed below.</P
><P
>In order to shut down a Linux system, first all processes
	are told to terminate (this makes them close any files and
	do other necessary things to keep things tidy), then filesystems
	and swap areas are unmounted, and finally a message is printed
	to the console that the power can be turned off.  If the proper
	procedure is not followed, terrible things can and will happen;
	most importantly, the filesystem buffer cache might not be flushed,
	which means that all data in it is lost and the filesystem on
	disk is inconsistent, and therefore possibly unusable.
	</P
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><P
>On early computers, it wasn't enough
		to merely turn on the computer, you had to manually load the
		operating system as well.  These new-fangled thing-a-ma-jigs do
		it all by themselves.</P
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